The present invention relates to copy protection, and, more particularly, to a method and system for copy-protecting a digital audio compact disc as well as to the resulting copy-protected compact disc.
Types of Compact Disc
The familiar compact disc (CD) has become one of the most highly successful of modem consumer products, with current annual worldwide production and distribution in the billions. Low manufacturing costs, availability of inexpensive recording and playback equipment, reasonably high data densities, extremely high reliability, noise immunity, absence of contact wear during playback, and versatility of the medium underlie the wide popular acceptance of the compact disc. There are two principal kinds of compact disc, distinguished by the nature of the recorded material:
The first kind of compact disc is termed the xe2x80x9caudio compact discxe2x80x9d, which herein denotes any compact disc on which audible sounds, such as music, speech, and other material in the audible spectrum can be recorded, and which substantially contains only information for reproducing audio signals. The audio compact disc is specified by the International Electrotechnical Commission (IEC) International Standard 908 xe2x80x9cCompact disc Digital Audio Systemxe2x80x9d, which is substantially the same as the original standard proprietary to Sony Corporation of Japan and Philips Electronics of the Netherlands. This standard is commonly known in the art, and denoted herein, as the xe2x80x9cRed Bookxe2x80x9d, and is incorporated by reference for all purposes as if fully set forth herein. The Red Book contains the basic physical specifications for the compact disc as well as the fundamentals of the optical readout of digital audio data therefrom by laser, the xe2x80x9ceight-to-fourteen modulationxe2x80x9d (EFM) data encoding scheme, the data interleaving, and a concise formulation of the mathematics of the xe2x80x9cCross Interleave Reed-Solomon Codexe2x80x9d (CIRC) digital error-correction method used to insure faithful sound reproduction in spite of scratches and other minor physical surface damage a compact disc can be expected to encounter during normal handling. More detailed information pertaining to the audio compact disc is available in popular publications, such as Principles of Digital Audio, by Ken C. Pohlman, published by McGraw-Hill, Inc., ISBN 0-07-0504687.
The basic Red Book format, recording, and playback processes for an audio compact disc are illustrated in FIG. 1 and FIG. 2, to which reference is now briefly made. In particular, the principal recording unit specified by the Red Book is a sector 150. Because sector 150 corresponds to a record data block 110 (FIG. 1) containing 2,352 usable bytes of data, the playback of the audio compact disc results in a playback data block 235 (FIG. 2) which also contains 2,352 bytes of audio data. The Red Book standard parameters are such that 75 consecutive sectors of audio data represent one second of audio signal content, and at 2,352 bytes per sector this results in a data rate of 176,400 bytes per second. The Red Book specifies that each audio data sample contains 2 bytes (16 bits), and that two audio channels are simultaneously encoded for stereo. Thus, each channel has an audio data rate of 44,100 samples per second, corresponding to an upper frequency of 22 kHz according to the well-known Nyquist criterion. The 16-bit resolution provides a dynamic range in excess of 90 dB, from xe2x88x9232,768 to +32,767.
The second kind of compact disc is termed the xe2x80x9ccompact disc read-only memoryxe2x80x9d (CD-ROM), which herein denotes any compact disc oh which arbitrary digital data may be recorded. The data on a CD-ROM may represent audio information, but it may also represent images, video, graphics, text, executable computer programs and data therefor, as well as any other information which may be represented digitally. The CD-ROM is specified by ISO/IEC International Standard 10149 xe2x80x9cData Interchange on Read-Only 120 mm Optical Data Disksxe2x80x9d, which is substantially the same as the original standard proprietary to Sony Corporation of Japan and Philips Electronics of the Netherlands. This standard is commonly known in the art, and denoted herein, as the xe2x80x9cYellow Bookxe2x80x9d, and is incorporated by reference for all purposes as if fully set forth herein. The Yellow Book standard is based upon the physical and fundamental data format specifications of the Red Book, and contains specifications for additional data formatting, sector addressing, mode specification, byte scrambling, an additional two levels of Reed-Solomon Product Code (RSPC) error-correction, error-detection encoding, and byte swapping. More detailed information pertaining to the CD-ROM as well as to compact discs in general is available in popular publications, such as The Compact Disc Handbook by Ken C. Pohlman, published by A-R Editions, Inc., ISBN 0-895-79-300-8.
(Although the present invention pertains only to audio compact discs as specified by the Red Book, some details of the Yellow Book are herein presented so that the limitations of the prior art as well as the functioning of the copy-protection afforded by the present invention may be better appreciated. It is not necessary, however, to take into consideration any details specific to the Yellow Book in order to carry out the method according to the present invention or to produce an audio compact disc with copy-protection according to the present invention.)
There are a number of formatting variations covered by the Yellow Book, and there are derivative formats covered by related standards. A conceptual view of the Yellow Book recording process and format is illustrated in general in FIG. 3, to which reference is now briefly made. In summary, Yellow Book recording from a computer 305 starts with an arbitrary record data block 310 containing from 2,048 bytes to 2,324 bytes of any kind of binary data. The presence or absence of the optional features indicated in FIG. 3 depend on the specific mode in use, so that the actual number of bytes available for data varies according to the mode. Record data block 305 is given an optional additional error-correction 315, a byte scrambling 320, and a byte swapping 325, and is then formatted into a Yellow Book record sector 330 containing a total of 2,352 bytes, including the original data from record data block 310 and additional data such as header, a sector address 330-2, mode, and other optional information. The particular format of Yellow Book record sector 330 also depends on the details of the specific mode in use. Finally, Yellow Book record sector 330 is treated as a Red Book record data block, which is then recorded onto compact disc according to the Red Book standard. Playback of the recorded Yellow Book CD-ROM is illustrated in FIG. 4, and is essentially the reverse of the recording process shown in FIG. 3. A CD-ROM drive 400 reads a CD-ROM 440 via a laser reader 445 whose position relative to CD-ROM 440 is set by a sector selector 450 which receives an input from a computer 435. A Red Book playback data block 405 is output from laser reader 445 and treated as a Yellow Book playback sector 410, which undergoes a byte de-swapping 415, a byte descrambling 420, and an optional error-correction 425 to yield the arbitrary binary data of playback data block 430 for computer 435. At the same time, sector address decoding 455 provides a reference input to sector selector 450.
Recording an Audio Compact Disc
FIG. 1 illustrates the basic prior-art recording process for an audio compact disc. An audio sampling source 105 generates pairs of 16-bit samples of a two-channel (stereo) audio signal at a rate of 44,100 samples per second. These create a record data block 110 containing 2,352 bytes of data every {fraction (1/75)} second, which are then encoded by an EFM encoding 115 as specified in the Red Book and as is well-known in the art. The bytes then undergo a crossing and cross-delay 120 followed by a computation 125 of a C2 error-correction codeword. This is followed by a C2 interleaving delay 130 and a computation 135 of a C1 error-correction codeword with a C1 interleaving delay. Details of crossing and cross-delay 120, C1 codeword and C1 interleaving delay, and C2 codeword and C2 interleaving delay, as well as the contents of a C1 codeword and C2 codeword are specified in the Red Book and are illustrated in FIG. 11 and FIG. 12. The purpose of the interleaving delays is to spread out the data content of the codewords over a relatively large physical area, and thereby prevent localized damage to the compact disc from corrupting more than a small portion of a codeword. A small amount of damage to a large number of codewords is correctable, but a large amount of damage to a small number of codewords is not. At the same time, A control and display encoding and synchronization pattern is generated in a computation 160, and combined with the interleaved codewords by a multiplexer 180. The result of the foregoing operations is the construction of a frame 140 containing a set of 33 EFM symbols, a typical symbol of which is indicated as a symbol 145. The 33 EFM symbols include a control symbol 140-2, a first set of 12 data symbols 1404, a set of four Q-parity symbols 140-6, a second set of 12 data symbols 140-8, and a set of four P-parity symbols 140-10. Note that frame 140 contains a total of 24 data symbols. Note that Q-parity symbols 140-6 are also denoted herein as C2 parity symbols and that P-parity symbols 140-10 are also denoted herein as C1 parity symbols.
The principal data storage unit is a sector 150, which contains a set 1504 of 98 consecutive and contiguous frames preceded by a synchronization header 150-2. Each sector contains 2,352 bytes of data corresponding to the 98 frames 1504 of 24 data bytes each. For every second of audio signal, 75 sectors are recorded onto an audio compact disc 155 (or a master) by a laser recorder 175 which is controlled by a linear tracking signal 170.
Error-Correction
The prior-art error-correction method utilized according to the Red Book standard consists of two levels of Reed-Solomon error-correction, which is well-known in the art. Each level of Reed-Solomon error-correction is based on a codeword containing a set of parity symbols in addition to a set of data symbols. The first codeword computed during the Red Book recording process, as illustrated in FIG. 1 is the C2 codeword, which contains 4 C2 parity symbols 140-6 in addition to 24 data symbols 140-4 and 140-8, for a total of 28 symbols. The second codeword computed during the Red Book recording process, as also illustrated in FIG. 1, is the C1 codeword, which contains 4 C1 parity symbols 140-10 in addition to the 28 symbols of the previously-computed C2 codeword, for a total of 32 symbols. Control symbol 140-2 is not part of any codeword and therefore is not protected by any error-correction.
Parity symbols represent redundancy within the codeword, and allow a certain amount of error to be corrected. As is well-known in the art, it requires 2 redundant symbols to correct a single arbitrary erroneous symbol (one whose position in the codeword is not known in advance). The use of the 2 redundant symbols allows computing the position of the arbitrary erroneous symbol and the magnitude of the error, and thereby allows that arbitrary erroneous symbol to be corrected (an arbitrary error in a parity symbol itself is also correctable). An erroneous symbol whose position is known in advance is termed an xe2x80x9cerasurexe2x80x9d, and requires only a single redundant symbol for correction, because the location of such a symbol is known and only the magnitude of the error needs to be computed. A measure of the total error of a codeword is therefore given by the number of erasures plus 2 times the number of arbitrary erroneous symbols. A Reed-Solomon codeword having 4 parity symbols therefore has a limit of correcting measure of 4. Thus, a C1 or C2 codeword according to the Red Book can be corrected for up to 4 erasures, or up to 2 arbitrary erroneous symbols, or 1 arbitrary erroneous symbol and up to 2 erasures. Any error condition in excess of these limits results in an uncorrectable error. For example, 3 arbitrary erroneous symbols, or 5 erasures, or 1 arbitrary erroneous symbol and 3 erasures, or 2 arbitrary erroneous symbols and 2 erasures are all examples of uncorrectable errors in C1 and C2 codewords.
Compact Disc Players and Drives
The term xe2x80x9caudio playerxe2x80x9d herein denotes any device for playing back the audible sounds recorded on an audio compact disc. Audio players include, but are not limited to, components for home entertainment systems, portable personal listening devices, entertainment systems for vehicles, and so forth. Audio players are often equipped with speakers or headphones so that they may be used as stand-alone devices for directly reproducing the sounds recorded on an audio compact disc without the need for any other equipment.
FIG. 2 illustrates the basic prior art arrangement for playback of audio compact disc 155 in an audio player 200. The playback process is essentially the reverse of the recording process illustrated in FIG. 1, with some important additions in connection with error-concealment. Audio player 200 reads audio compact disc 155 via a laser reader 270 whose positioning relative to the data on audio compact disc 155 is controlled according to a track selection 265, which receives input from a sound system 250. The audio data samples output from laser reader 270 are read as sectors, such as sector 150, which is then interpreted as a sequence of frames, such as frame 140. Information contained in frame 140 is separated by a multiplexer 255 into a main data channel stream that progresses to C1 codeword processing in a step 205, and a subcode channel that progresses to control and display processing in a step 260. Audio player 200 reads 75 sectors per second in accordance with the original audio signal sampling. During playback, the C1 codeword is examined first in step 205 to detect isolated errors and apply correction. As is known in the art, C1 decoders are usually set to correct at most a single arbitrary erroneous symbol and therefore are able to detect error conditions in excess of this limit accurately, and to pass along error-detection information to the C2 decoder from step 205 to a step 220. The C2 codeword is prepared by a C2 interleave advance 210. At the C2 codeword decoding stage in step 220, a detected error within the error-correction limits, as described above, results in the correction of the errors. However, a detected error in excess of the error-correction limits, as described above, results in the generation of what is termed in the art as an xe2x80x9cE32 errorxe2x80x9d by an E32 error detector 240. An E32 error signifies that an uncorrectable error has been detected.
If no errors are detected or if all detected errors can be corrected, the audio signal is reassembled by a cross advance/decrossing 225 and an EFM decoding 230. The reassembled audio signal data is then presented in playback data block 235 for reproduction as an audio signal in sound system 250. If, however, uncorrectable errors are detected by E32 error detector 240, an error-concealment unit 245 hides the uncorrectable errors by performing an interpolative error-concealment, as is illustrated in FIG. 10, and as is discussed below. Through this means, the vast majority of errors present on audio compact disc 155 are either corrected or concealed, with the result that the reproduced audio signal usually appears to be error-free to the listener. The term xe2x80x9cCD-ROM drivexe2x80x9d herein denotes any device which is able to read the arbitrary digital data recorded on a CD-ROM. CD-ROM drives are not used as stand-alone devices by themselves, but rather as components within a computer system, which make the data recorded on a CD-ROM available to the computer system. Typically, a CD-ROM drive is also able to read the audio signal data recorded onto a Red Book audio compact disc. The basic prior art arrangement for accessing the data of a compact disc 510 via a CD-ROM drive 505 for a computer is illustrated in FIG. 5. Note that compact disc 510 can be an audio compact disc or a CD-ROM. First, compact disc 510 is read by laser reader 445, whose position relative to compact disc 510 is controlled by a positioning unit 570, which receives an input from a computer data bus 530. Data from laser reader 445 is input to a Red Book decoder 515, which obtains a Red Book playback data block 405 (FIG. 4), which is passed to a Yellow Book decoder 520. If Red Book playback data block 405 corresponds to a Yellow Book playback sector 410 (FIG. 4) then Yellow Book playback sector 410 is processed to extract a playback data block 430 (FIG. 4) which is presented as computer data interface input 525 to a computer data bus 530. If, however, Red Book playback data block 405 does not correspond to a Yellow Book playback sector, then Yellow Book decoder 520 may optionally ignore Red Book playback data block 405, or may optionally pass Red Book playback data block 405 without Yellow Book processing as computer data interface input 525 to computer data bus 530. In this latter case, computer data interface input 525 corresponds to audio signal data. Furthermore, if Red Book playback data block 405 does not correspond to a Yellow Book playback sector, then Red Book decoder 515 sends the audio output 535 corresponding to Red Book playback data block 405 to an error-concealment unit 580 for output to a sound reproduction device 545. As with the playback of an audio compact disc by an audio player as discussed above. error-concealment unit 580 prevents the output of uncorrectable errors. Note that positioning unit 570 receives reference input from both Red Book decoder 515 and Yellow Book decoder 520. If compact disc 510 is an audio compact disc, then the reference input to positioning unit 570 will be control and display information 550 from Red Book decoder 515. If, however, compact disc 510 is a CD-ROM, then the reference input to positioning unit 570 will also include Yellow Book sector address information 560, as illustrated in Yellow Book playback sector 410 (FIG. 4) It is important to emphasize that if compact disc 510 is an audio compact disc, only control and display information 550 from Red Book decoder 515 will be available as reference input to positioning unit 570.
It is important to note that neither the Red Book nor the Yellow Book, nor any other official standards for the compact disc specify any standards governing the design, operation, or performance of audio players or CD-ROM drives. The published standards are concerned only with the compact disc media itself and not with any devices used to make compact disc recordings or to play back material recorded on compact disc. There is therefore some variation in the performance, data handling, and error-correction capabilities of various audio players and CD-ROM drives. It is expected, however, that any commercially-available audio player, CD-ROM drive, or CD recorder will be able to play and/or record compact discs according to the published standards. In addition, the competitive nature of the market dictates that certain public expectations regarding the performance of such devices will be normally met. Thus, there are various principles and criteria for audio players and CD-ROM drives which are generally accepted and applied throughout the industry. These principles and criteria are discussed in various publications such as the previously cited books by Ken C. Pohlman. Such performance criteria include the ability of audio players to perform error-concealment during playback.
As noted above, because the CD-ROM Yellow Book standard is based on the audio compact disc Red Book standard, CD-ROM drives are also generally able to play audio compact discs. It is important to emphasize, however, that in the majority of commercially-available CD-ROM drives, the output of audio data from an audio compact disc is done through audio output 535, which is separate from computer data interface input 525 that makes the data accessible for computer use on computer data bus 530, as shown in FIG. 5. This separation of the audio and data channels is necessary because arbitrary data recorded on a CD-ROM must undergo the additional processing of Yellow Book decoder 520 in order to be available for computer use. As will be detailed subsequently, a result of this separation of audio and data channels is that for most commercially-available CD-ROM drives, computer data interface input 525 available for computer use does not necessarily undergo exactly the same processing as the audio output 535 which may be played by the CD-ROM drive. In particular, for most commercially-available CD-ROM drives, computer data interface input 525 for computer use on computer data bus 530 does not undergo error-concealment processing in error-concealment unit 540, and, as discussed in detail below, this is a fact that can be exploited to impart copy-protection to an audio compact disc according to the present invention.
Compact Disc Production Methods
There are two general methods utilized for producing compact discs. These methods apply equally to both audio compact discs and CD-ROM""s. One method is well-suited to large-scale production, whereas the other method is better-suited to small-scale production.
The large-scale production method for compact discs involves molding the finished disc in a mold created from a master. Compact discs produced by this first method are herein termed xe2x80x9cstamped discsxe2x80x9d. The production of stamped discs is characterized by a high setup cost, with a low unit cost and high output rate. Virtually all commercially-available compact discs are stamped discs.
The low-scale production method for compact discs involves recording each disc individually in a device denoted herein as a xe2x80x9cCD recorderxe2x80x9d using special recordable compact disc media. Compact discs produced by this second method are herein termed xe2x80x9crecorded discsxe2x80x9d. The production of recorded discs is characterized by a negligible setup cost, with a relatively high unit cost and a very low output rate. Virtually all compact discs produced by individuals and other end-users are recorded discs.
Unauthorized Copying of Compact Discs
Because the data recorded on compact disc is in a digital format with an error-correction capability, it is possible to make faithful copies whose playback is indistinguishable from that of the original disc from which the copy was made. Moreover, a digital copy made from such a digital copy is also generally indistinguishable from the original, in contrast to analog recording, where repeated copying results in a gradual, but discemable, deterioration of the original quality.
Furthermore, because of the widespread commercial success of the compact disc and the standardization of the media, equipment for producing compact discs is readily-available and relatively inexpensive, both for stamped discs and for recorded discs. As a result, many organizations as well as individuals are capable of making copies of compact discs, and the unauthorized or illegal copying of compact discs has thus become a serious problem. Means for combating the unauthorized copying of compact discs include legal action to enforce copyright laws as well as technological methods to render it difficult or impossible to make an unauthorized copy of a compact disc. Such technological methods fall under the general category of xe2x80x9ccopy-protectionxe2x80x9d methods, which is a term used herein for describing any methods which allow an original recording to be utilized, but which prevent unauthorized copies of the original from being made, or which render such unauthorized copies substantially unusable. The process of applying copy-protection is herein denoted by the term xe2x80x9ccopy-protectingxe2x80x9d, and any original medium so processed is herein denoted by the term xe2x80x9ccopy-protectedxe2x80x9d.
Unauthorized copying of compact discs by mass-producing stamped disc copies is done by well-funded organizations that are skilled in the technology of compact discs. The resulting unauthorized copies can thus be made to look identical to the original, and can therefore be sold for profit into the legitimate market and purchased by unsuspecting distributors and consumers. Although the organizations responsible for manufacturing such unauthorized copies may have a visible public profile because of the facilities and staff required for their operations, they are usually located where legal enforcement of copyright laws is lax. Copy-protection measures for thwarting unauthorized copying of stamped discs is hampered by the fact that these organizations are technologically sophisticated and are usually capable of disabling copy-protection. The most effective way to combat such mass-produced unauthorized copying is through expanded enforcement of the copyright laws, and this is the route that is currently being taken by the recording and publishing industries.
Unauthorized copying of compact discs by individually-producing recorded discs, however, is being done with increasing frequency by ordinary consumers who have purchased inexpensive computer systems and CD recorders. Such systems and the copying software for reproducing compact discs are easily obtained and operated. The resulting unauthorized copies are readily distinguishable visually from the original, and therefore cannot be sold in the marketplace for a profit as can the unauthorized stamped discs described above. These unauthorized recorded disc copies, however, are informally distributed for personal use by social contact, and displace the sale of legitimate compact discs. As computer systems and CD recorders continue to decline in price and increase in numbers throughout society, unauthorized copying of compact discs by individually-producing recorded discs is becoming a more and more serious problem. Because this unauthorized copying can be done by ordinary consumers working unassisted in complete privacy, it is impossible to control by means of law enforcement. Only suitable copy-protection methods can succeed in reducing the increasing flood of these unauthorized recorded disc copies. Unfortunately, as detailed below, the existing prior art copy-protection methods are unsuitable and/or inadequately effective for the audio compact disc.
Copying a Compact Disc
FIG. 6 illustrates a general method of copying original compact disc 510 using a small computer system equipped with CD-ROM drive 505 and a CD recorder 600. This method generally applies whether original compact disc 510 is an audio compact disc or a CD-ROM. CD-ROM drive 505 reads the data content of compact disc 510 and places computer data interface input 525 onto computer data bus 530. Through the use of copying software 605 the data of original compact disc 510 appears as computer data interface output 610 to a Yellow Book encoder 620, which is part of CD recorder 600. If computer data interface output 610 corresponds to Yellow Book data (where original compact disc 510 is a CD-ROM), then Yellow Book encoder 620 processes computer data interface output 610 and then passes Yellow Book-encoded data to a Red Book encoder 625 for recording onto compact disc copy 630, according to the recording method for a CD-ROM as illustrated in FIG. 3 (as before, there is a laser recorder 635 whose position is controlled by a linear tracking 640). Otherwise, if computer data interface output 610 corresponds to Red Book data (where original compact disc 510 is an audio compact disc), then Yellow Book encoder 620 does not process computer data interface output 610, but simply passes the audio signal data to Red Book encoder 625 for recording onto compact disc copy 630, according to the recording method for an audio compact disc as illustrated in FIG. 1. In either case, compact disc 630 is a copy of original compact disc 510. Note that copying software 605 sends positioning information to positioning unit 570 to control laser reader 410. As previously noted, if original compact disc 510 is a CD-ROM, then positioning unit 570 receives reference input both from Yellow Book decoder 520 (in the form of sector address information 560) and Red Book decoder 515 (in the form of control and display information 550), but if original compact disc 510 is an audio compact disc, then positioning unit 570 receives reference input only from Red Book decoder 515 (in the form of control and display information 550). The proper positioning is necessary to assure that copying software 605 receives the necessary data during the copying process.
Prior Art Copy-Protection Schemes
There are a number of existing copy-protection schemes specifically for CD-ROM""s, which fall into several classes, most of which rely on the fact that the protected CD-ROM must be used with a computer. A first class of CD-ROM copy-protection scheme involves recording special computer software on the protected CD-ROM in conjunction with the placement of special marks on the original CD-ROM, such that these marks can be read by an ordinary CD-ROM drive, but such that these marks cannot be duplicated by an ordinary CD recorder. The special software recorded on the protected CD-ROM is necessary for utilizing the data or other information also recorded thereon, and also checks for the presence of the special marks. If the special marks can be read, the software considers the compact disc to be an original, and allows access to the data and information recorded thereon. Otherwise, if the special marks cannot be read, the software considers the compact disc to be an unauthorized copy, and does not allow access to the data and information recorded thereon. An example of such a copy-protection scheme is disclosed in U.S. Pat. No. 5,809,006 to Davis et al. (herein referred to as xe2x80x9cDavisxe2x80x9d), which records special marks on a compact disc in the form of a radial track wobble or channel clock rate variations. A second class of CD-ROM copy-protection scheme involves encrypting the data recorded on the CD-ROM, and providing the decryption key in a manner that is independent of the copied data. Schemes of this second class can be combined with those of the first class by encoding the decryption key within the special, non-copyable marks or alternative physical characteristics of the compact disc. Examples of this are disclosed in U.S. Pat. No. 5,923,754 to Angelo, et al. (herein referred to as xe2x80x9cAngeloxe2x80x9d), and in U.S. Pat. No. 5,915,018 to Aucsmith (herein referred to as xe2x80x9cAucsmithxe2x80x9d). Because all of these schemes require the involvement of a computer or other programmable device to determine whether access to the recorded information is permitted, none of them is suitable for providing copy-protection to an audio compact disc that is to be played by an ordinary compact disc audio player. A third class of CD-ROM copy-protection scheme relies on modifying the data format of the protected CD-ROM, in such a way that recording devices and/or recordable compact disc media cannot accommodate the modified format. An example of such a scheme is disclosed in U.S. Pat. No. 5,832,088 to Nakajima et al. (herein referred to as xe2x80x9cNakajimaxe2x80x9d), which utilizes an abnormally-long data length that exceeds the capacity of the recordable compact disc media while relocating retrieval data so that it normally will not be copied. Such schemes also cannot be used for copy-protection of audio data because the principles upon which they rely are specific to data access, and are not applicable to audio playback.
There are furthermore various copy-protection schemes which are specific to other kinds of digital optical media. Examples of this are disclosed in U.S. Pat. Nos. 5,699,434 and 5,828,754 both to Hogan as well as the Angelo and Aucsmith patents previously cited, all of which pertain specifically to the unique digital encoding and formatting scheme of the Digital Video Disk (DVD), and whose methods are not applicable to the audio compact disc, whose encoding and formatting does not have the properties required to use these methods.
There also are a number of existing copy-protection schemes specifically for compact discs that are used with special players or other special equipment. One class of copy-protection scheme of this sort involves making changes in the format of the data recorded on the compact disc, such that the compact disc cannot be read by an ordinary CD-ROM drive and therefore cannot be copied using such a device. Another class of such schemes does not directly prevent the making and using of unauthorized copies, but rather provides only a special method and device for differentiating between an original compact disc and an unauthorized copy. An example of this is disclosed in U.S. Pat. No. 5,696,757 to Ozaki et al. (herein referred to as xe2x80x9cOzakixe2x80x9d). Still another class of scheme involves a central database which is automatically contacted by the special player or special equipment in conjunction with each use of the material recorded on the protected compact disc. The central database maintains accounting information and bills the user for each use of the protected material. Use of the material is restricted by the special player or special equipment to that which is authorized by the central database. Strictly speaking, this is not a copy-protection method, but a usage-control system. Because these schemes require the use of special players or other special equipment, they are not suitable for providing copy-protection to an audio compact disc that is to be played by an ordinary compact disc audio player and which is vulnerable to being copied by ordinary computer equipment and CD recorders.
Another class of copy-protection scheme applies to recorded audio signals regardless of the medium. Schemes of this class involved masking the recorded audio signals with noise that is filtered out by special playback equipment. An example of such a scheme is disclosed in U.S. Pat. No. 5,394,274 to Kahn (herein referred to as xe2x80x9cKahnxe2x80x9d), which discloses a device, to be incorporated into consumer equipment, which removes the superimposed noise for playback, but which restricts the copying thereof. Yet another class of copy-protection scheme requires the use of special playback/recording equipment that can be selectively unlocked to allow playback and selectively locked to prevent recording. An example of such a scheme is disclosed in U.S. Pat. No. 4,979,210 to Nagata et al. (herein referred to as xe2x80x9cNagataxe2x80x9d), which places supplemental information on the protected audio recording and relies on a detector to detect this supplemental information and disable the copying device. Another example of such a scheme is disclosed in U.S. Pat. No. 5,083,224 to Hoogendoom et al. (herein referred to as xe2x80x9cHoogendoomxe2x80x9d), which places a substantially inaudible copy-protection code in a low-frequency differential signal between the audio stereo channels, and which is detected by special playback/recording equipment to disable or distort the recording. Because all schemes of this sort rely on special playback and/or recording equipment, they are unsuitable for providing copy-protection to an audio compact disc that is to be played by an ordinary compact disc audio player and which is vulnerable to being copied by ordinary computer equipment and CD recorders.
A copy-protection method specific to compact disc which is applicable both to CD-ROM as well as audio compact discs is disclosed in U.S. Pat. No. 5,930,209 to Spitzenberger et al. (hereinafter referred to as xe2x80x9cSpitzenbergerxe2x80x9d), and relies on placing irregularities in sector addresses to prevent unauthorized copying by interfering with the reference information input to positioning unit 570 (FIG. 6). Unfortunately, this method is not entirely successful in preventing unauthorized copying of audio compact discs, because many CD-ROM drives and types of copying software do not need to continuously control positioning unit 570 when copying audio data tracks, and thus irregularities in sector addresses as disclosed by Spitzenberger have no deterrent effect on such CD-ROM drives.
A recently-published announcement on the Internet by C-Dilla in England (xe2x80x9cC-Dilla Announces AudioLok,xe2x80x9d Jul. 28, 1999) alleges that the error-correction codes for a Red Book audio compact disc are different from those of a Yellow Book CD-ROM, and that it is therefore possible to manipulate the error-correction codes of an audio compact disc to prevent the audio compact disc from being readable by a CD-ROM drive. This, however, is misleading, in that the error-correction codes specified for Red Book audio compact discs are absolutely identical to the principal error-correction codes specified for Yellow Book CD-ROM""s, as is illustrated herein in FIG. 3 and FIG. 4. Methods of manipulating these error-correction codes to prevent a compact disc from being readable by a CD-ROM drive are known in the art, including methods as previously disclosed in U.S. patent application Ser. No. 09/032,905 dated Mar. 2, 1998, which is based on provisional application No. 60/038,080 dated Mar. 6, 1997 to one of the present inventors. Moreover, it has been discovered that while manipulating the error-correction codes of an audio compact disc renders that audio compact disc partially unreadable by some CD-ROM drives, there are nevertheless many ordinary CD-ROM drives available on the consumer market which are capable of reading such an audio compact disc, sometimes through the use of special software commands. The method described in the above-cited published announcement by C-Dilla, therefore, does not represent as good a form of copy-protection as is possible to attain.
There is thus a widely recognized need for, and it would be highly advantageous to have, a means of copy-protecting a digital audio compact disc such that ordinary audio players may be utilized for playback of the original disc without relying on the use of any special equipment for playback, and which will enforce the copy protection when an attempt is made to copy the original disc by reading it using an ordinary CD-ROM drive with commercially-available compact disc recording equipment. This goal is met by the present invention.
The present invention relies upon the fact that the audio compact disc has been designed to reproduce sounds as accurately as possible in the face of error conditions, and in addition to having two levels of error-correction, is designed to facilitate the use of error-concealment during playback. The opportunity to use error-concealment is unique to the audio compact disc, and is not available for other optical disc media, such as the DVD, nor is it applicable to most data stored on a CD-ROM.
Errors can arise through physical damage to a compact disc such that an original data value of the audio signal is obscured or obliterated. The Red Book specifies storing two levels of additional redundant parity information with the audio signal for error-correction under such conditions. Normally, therefore, occasional errors can be corrected by the audio player utilizing this parity information in accordance with mathematical error-correction techniques that are well-known in the art and which are specified in the Red Book standard. It can happen, however, that a serious error can occur, which is beyond the mathematical ability of the error-correction codes to repair. Such an error is herein denoted by the term xe2x80x9cuncorrectable errorxe2x80x9d. With error-concealment, an audio player will hide the uncorrectable error by substituting an interpolated value for the audio data sample in place of the erroneous value. In most cases, the substituted interpolated value will closely approximate the correct value, because of the 44.1 kHz sampling rate. Thus, for all but the highest-frequency components contained in the original audio signal, the interpolated value will be audibly indistinguishable from the correct values. (This can be seen by considering the case where half of the audio data samples are replaced by interpolated values, which would be equivalent to sampling at half the 44.1 kHz frequency. The resulting upper audio frequency would be 11 kHz, which still encompasses most of the audible spectrum. An occasional application of error-concealment, therefore, will not audibly degrade the quality of the audio compact disc.) Consequently, the intentional insertion of an uncorrectable error generally is not audible, and moreover, in accordance with the preferred embodiment of the present invention, it is possible to select specific audio data samples for replacement by erroneous values in such a way that the error-concealments are definitely inaudible. (As noted previously, although the format of the audio compact disc was designed to facilitate error-concealment, and all currently-available audio players implement error-concealment, the process and implementation of error-concealment is not covered in the Red Book and is not part of the official specification of the audio compact disc.)
In a first step of the preferred embodiment of the present invention, erroneous symbols (that is, symbols having erroneous values) are deliberately recorded on the audio compact disc in place of certain correct values of the audio data samples, in such a way that upon playback, the erroneous values would normally reproduce an objectionable noise superimposed over the correct audio signal. The error-correction information recorded on the audio compact disc, however, would normally allow these erroneous values to be perfectly corrected during playback so that the original correct audio signal would be output. Therefore, in a second step of the preferred embodiment, the error-correction information which corresponds to the erroneous values is also overwritten with invalid values such that the audio player will be unable to perform the error-correction. Nevertheless, even though the errors are not correctable, the audio player detects the presence of these erroneous values and hides them by error-concealment, as described above. Thus, upon playback of an audio compact disc according to the present invention, the substituted erroneous values will be inaudible when played on an ordinary compact disc audio player.
If, however, an attempt is made to copy the compact disc using an ordinary computer system having a CD-ROM drive for playing the original compact disc, a CD recorder for recording a copy under control of copying software, in the majority of cases the CD-ROM drive will not apply error-concealment to the data read from the compact disc, but will either fail to read the erroneous values (because they are uncorrectable errors) or will simply read the erroneous values in the audio signal and send these erroneous values to the CD recorder for copying onto the recordable compact disc media. Thus, either substantial portions of the original compact disc will be uncopyable because of the uncorrectable errors, or the erroneous values of the original will be copied. In the former case, the copy will reproduce silence in place of substantial portions of the original audio signal, and in the latter case, the copy will reproduce objectionable noise in place of substantial portions of the original audio signal. In either of these cases, the copy will be rendered substantially unusable, whereas the original compact disc will faithfully reproduce the original audio signal when played in an ordinary CD audio player.
Thus, the method according to the present invention and the audio compact disc thereof implement copy-protection through the use of latent noise, which does not appear when the original audio compact disc is played on an ordinary compact disc audio player, but which is either manifest in unauthorized copies of the audio compact disc made on a computer system having an ordinary CD-ROM drive and an ordinary CD recorder, or which disrupts the recording of an unauthorized copy of the audio compact disc. That is, the latent noise does not interfere with the playback of the copy-protected audio compact disc on an ordinary audio player, but the latent noise does interfere with the unauthorized copying of the audio compact disc on an ordinary CD recorder and with the playback of unauthorized copies made on ordinary CD recorders.
To augment the protection provided by the latent noise method, where a CD-ROM drive employs error-concealment on audio signals read as data, the method of the present invention also selectively disables the control and display reference information to the laser reader positioning unit. This is accomplished by manipulating the subcode channel of the audio tracks.
This copy-protection is enforced by the limitations of the ordinary CD-ROM drive used to read the original compact disc, and does not depend on the use of special recording or playback equipment. Furthermore, such copy-protection according to the present invention cannot be disabled or evaded by the copying software.
Therefore, according to the present invention there is provided a method for producing a copy-protected audio compact disc containing a plurality of symbols within error-correction codewords representing audio data samples of an audio signal, by including latent noise on the copy-protected audio compact disc which does not interfere with the playback of the audio signal from the audio compact disc on an ordinary audio player, but which interferes with the unauthorized copying of the audio compact disc on an ordinary CD recorder and with the playback of an unauthorized copy of the audio compact disc made on an ordinary CD recorder, the method including the steps of: (a) selecting at least one audio data sample of the audio signal; (b) locating the data symbols representing the at least one audio data sample; (c) overwriting the data symbols with erroneous symbols; (d) locating the error-correction codewords which contain the data symbols; and (e) disabling the error-correction of the error-correction codewords.
Furthermore, according to the present invention there is provided a copy-protected audio compact disc containing a plurality of symbols representing audio data samples of an audio signal, and including latent noise which does not interfere with the playback of the audio signal from the audio compact disc on an ordinary audio player, but which interferes with the unauthorized copying of the audio compact disc on an ordinary CD recorder and with the playback of an unauthorized copy of the audio compact disc made on an ordinary CD recorder, the copy-protected audio compact disc including at least one erroneous symbol that does not correspond to the audio signal, and wherein the at least one erroneous symbol is contained within a disabled error-correction codeword.
Moreover, according to the present invention there is provided a system for producing a copy-protected audio compact disc containing a plurality of symbols within error-correction codewords representing audio data samples of an audio signal, for including latent noise on the copy-protected audio compact disc which does not interfere with the playback of the audio signal from the audio compact disc on an ordinary audio player, but which interferes with the unauthorized copying of the audio compact disc on an ordinary CD recorder and with the playback of an unauthorized copy of the audio compact disc made on an ordinary CD recorder, the system including: (a) an audio data source providing audio data samples to be recorded onto a copy-protected audio compact disc; (b) a Red Book encoder for encoding the audio data samples; (c) a noise generator for generating the latent noise; (d) a codeword disabling unit for disabling error-correction codewords; (e) a recording laser; (f) a laser controller for controlling the recording laser; (g) a switch for selecting between the output of the Red Book encoder and the output of the codeword disabling unit, and for passing the selected output to the laser controller.